The following background is for convenience of those skilled in the art and for incorporating the listed citations by reference. The following is not an assertion that a search adequate for examination has been made, or that no other pertinant art exists, or that any of the following citations are material, or that any cf the following citations are analogous art, or prior art.
Solder joints are a critical part of surface mount attachment of electronic components. Previously, process quality of SMT circuit card assembly was monitored using periodic visual inspections, pull tests, and accelerated thermal cycling (ATC). Visual inspection of surface mount components SMCs is not sufficient because many defects are not visible. For example, inner joints of ball grid array BGA type SMCs cannot be observed readily. Also, some solder joint defects such as closed cracks, cold joints, and voids cannot be seen.
In pull testing, assembled components are pulled off the circuit card and the appearance of the fractured joints indicates the quality of the assembly process. Again, this method does not reveal all defects because it does not replicate field failure mechanisms; that is, joints do not pull apart in the field, but generally fail from fatigue due to shear stress resulting from different temperatures and/or different coefficients of thermal expansion. During pull testing, joints may pull apart at points far from cracks or voids at which fatigue would cause the joint to fail. Both visual inspection and pull testing require extensive services of a skilled operator for each board assembly tested.
ATC is a good method of finding defects that could cause field failures. Also, ATC testing can be automated so that a skilled operator is only required to electrically test the board after cycling and possibly periodically during cycling. However, ATC is not a practical testing method for monitoring continuous assembly process quality because the total test time is too long. Typically, ATC data is not available until weeks after production, and sometimes tests require months to complete. The cycle time for ATC testing is limited by the heat capacity of the contents of the chambers as well and the heating and cooling capability of the equipment; furthermore, very rapid heating or cooling can substantially affect the results by causing strains between portions with different heat capacities and limiting the creep time at operating temperatures. For interconnect card assemblies, cycle times are typically 20 to 40 minutes.
ATC testing has been used to screen out assemblies subject to early failures during so called burn-in, in which the product is cycled. Some joints which shear due to fatigue remain in compression throughout thermal cycles. If cracks through the joint are under compression during thermal cycling they cannot be detected by functional or continuity testing; these types of failures, however, tend to become evident under field conditions, since the confronting surfaces of the crack will oxidize over time, resulting in field failures. The cycle count of failures during ATC testing is difficult to determine because some failures are difficult to detect during ATC testing. Typically, during product development to prove designs, ATC testing is carried out a predetermined number of cycles and then pull tests are used to find any failures continuity testing would miss.
Those skilled in the field of the invention are directed to the following citations published before the filing date of this application, which may provide guidance in building card twist test machines, processes for testing circuit card assemblies, and processes for analyzing results. Japanese Patent JP 03-245600 and Soviet Union Patent SU 1723679-A1 disclose mechanical testing of joints of circuit card assemblies. In other fields, "U.S. Pat. No. 3,381,526 to Rastogi; U.S. Pat. No. 3,665,751 to Paine; U.S. Pat. No. 3,910,105 to Hoffstedt; U.S. Pat. No. 4,003,247 to Moser U.S. Pat. No. 4,567,774 to Manahan et al."; U.S. Pat. No.4,777,829 to Fleischman; U.S. Pat. No. 4,895,027 to Manahan; U.S. Pat. No. 4,958,522 McKinlay; U.S. Pat. No.5,079,955 to Eberhardt; U.S. Pat. No. 5,184,517 to Kelzer disclose test machines, processes or analysis of results.
All the above citations are hereby incorporated in whole by reference.